First, do no harm

Aulacaspis yasumatsui Delivers a Blow to International Cycad Horticulture

The literature covering the biology, invasion chronology, host plant responses, and control efforts of the armored scale Aulacaspis yasumatsui Takagi (Hempitera: Diaspididae) is reviewed. The small size of this cycad pest and complex surface morphology of the host cycad organs combine to make visual detection of every cryptic infestation difficult or impossible to achieve. The international movement of Cycas revoluta Thunb. nursery plants and the presence of C. revoluta nursery industries in so many countries have enabled this pest to wreak havoc on the international cycad horticulture trade over the last 25 years. The short pre-oviposition period and considerable female fecundity lead to rapid population expansion on the plants initially infested in newly invaded regions. A depletion of non-structural carbohydrates accompanies long-term infestations and precedes plant death. Enemy escape within the invasive range allows the scale population growth to remain unchecked until anthropogenic efforts establish non-native biological control.

Reciprocal Garden Study Reveals Acute Spatial-Edaphic Adaptation for Cycas micronesica

A long-term reciprocal garden study was used to determine adaptive variation between Cycas micronesica K.D. Hill plants from north versus south Guam. Half-siblings from each location were planted as one-leaf seedlings in north and south gardens and monitored for 15 years. Stem height and diameter, and leaf number and maximum length were measured yearly. Survival and plant size traits were evaluated using a two-way factorial. In both locations, the local genotypes out-performed the foreign genotypes in terms of survival and growth. Survival of the foreign genotypes began to decline by year 4 and was less than 10% by year 15. Survival of the local genotypes was 70% for the north garden and 100% for the south garden. The north site was more hostile to plant performance because overall survival and plant growth were less than for the south site. The most likely environmental factor provoking local adaptation was highly contrasting soil characteristics between north and south Guam. The results indicates that long-term conservation success for C. micronesica and other cycad species must include the concept of local adaptation into decisions for transplantation and restoration projects.

Biotic Threats to Cycas micronesica Continue to Expand to Complicate Conservation Decisions

Simple Summary

Effective conservation of endangered plant species requires identifying their greatest threats to formulate management protocols. Invasive species are a result of global change and are a major threat to biodiversity. We used the island cycad Cycas micronesica K.D. Hill as a model that represents the global issues of conservation science and invasion biology. In Guam, several non-native insect invasions began in 2003 and have combined to threaten the island population of this cycad species. In this article, we summarize the history of reported invasions and the reported non-native insect herbivores that have recently increased the threat status. We also discuss the interactions among herbivores that threaten the sustainability of C. micronesica on the island of Guam.

Abstract

Invasions of non-native species can threaten native biodiversity, and island ecosystems are ideal for studying these phenomena. In this article, first, we report on the invasive species that combine to threaten the island cycad Cycas micronesica by reviewing the history of previously reported invasions and providing an update of recent invasions. Then, we prioritize the threat status of each herbivore and the interactions among them. Plant damage was initiated in 2003─2005 by the non-native Aulacaspis yasumatsui Takagi armored scale, Erechthias sp. Meyrick leaf miner, and Luthrodes pandava Horsfield butterfly, which elicited unprecedented irruptions of the native Acalolepta marianarum Aurivillius stem borer and increased herbivory by feral pigs (Sus scrofa L.). The combined impact of these five consumers represents the greatest sustained threat to the cycad tree species. Mitigation of the damage caused by phytophagous non-native species is urgently needed to conserve this unique gymnosperm tree.

Height increment of Cycas micronesica informs conservation decisions

Growth dynamics of pachycaulous stems of arborescent cycad plants are not well understood, and most observations have been made in cultivated garden plants. We studied Cycas micronesica plants in Guam, Tinian, and Yap to understand the influences of geography, plant size, sex, and herbivory on stem growth. We also determined the changes in demography of Guam's population after 15 years of damage by non-native insect herbivores. The height increment (HI) was similar for plants within the height range from 100 cm to more than 600 cm, so the relative growth rate declined with height. Female tree HI was 68% of male tree HI, and Yap tree HI was 87% of Guam tree HI. Chronic herbivory by non-native insect herbivores caused a mean 44% decline in HI. Plants in managed gardens grew more rapidly than plants in a wild habitat. The HI was used to estimate that Guam has experienced a complete loss of ≈70 y of demographic depth resulting from the selective mortality of small plants since 2005. When future conservation interventions successfully mitigate the ubiquitous biological threats, our HI may be useful for empirically quantifying recovery of plant health.

Does Phytogeography Change with Shifts in Geopolitics? The Curious Case of Cycads in the United States

The United States is currently home to five native cycad species. We provide a discussion on these five cycad species to illuminate how evolutionary and geopolitical processes influence phytogeography and published checklists of threatened plants. The number of threatened species in need of protection within any given country is a product of speciation that is contingent with evolutionary processes. However, this number may change instantaneously along with shifting of geopolitical boundaries brought about by armed conflict between rival states and multilateral negotiations. There are five contemporary cycad species within the United States, and the various historical bilateral and multilateral agreements that have generated this list are reviewed. Three of these five cycad species are threatened and in need of urgent protection. A discussion on the history of United States cycads as a microcosm of worldwide conservation issues is presented, with a focus on how federal conservation endeavors of individual nations may influence the world’s biodiversity crisis.

Burrowing activity of coconut rhinoceros beetle on Guam cycads

Guam’s established population of non-native coconut rhinoceros beetle (CRB, Oryctes rhinoceros L.) began creating burrows in stem apices of several cycad species in a managed garden. We conducted an island-wide survey to determine the spatial patterns of CRB burrowing of stems of in situ Cycas micronesica. We also measured starch of healthy and unhealthy coconut leaf tissue and compared this with starch of cycad stem tissue. The starch concentration of the central unexpanded leaf in healthy Philippine coconut trees was ≈90 mg·g⁻¹, and that of unhealthy Guam coconut trees was ≈40 mg·g⁻¹. The starch concentration of the tissue within the CRB burrow locations on C. micronesica trees was 145 mg·g⁻¹. Burrowing of C. micronesica was restricted to female CRB adults and was found throughout the full latitudinal gradient of Guam. Our findings indicate Guam’s unhealthy coconut trees are no longer nutrient-dense, and the female CRB population may have exhibited a phylogenetically distant host shift to the abundant C. micronesica plants for a starch-rich diet within the concepts of the ‘ecological fitting’ hypothesis. We add proximity to coconut tree habitats as a new threat to Guam’s endangered C. micronesica population.

Longitude, Forest Fragmentation, and Plant Size Influence Cycas micronesica Mortality Following Island Insect Invasions

Island invasions may cause severe changes in biodiversity, but the factors that influence these changes are not well understood. We established 120 plots in Cycas micronesica habitats throughout Guam in 2005 following the invasion of the armored scale Aulacaspis yasumatsui, then observed plant mortality through 2020. We used transects in Yap as benchmarks, as the Yap C. micronesica population is not threatened. The initial Guam plots contained about 1600 seedlings, 1160 juveniles, and 1240 mature plants per ha. Seedling mortality was 100% by 2006, juvenile mortality was 100% by 2014, and the 2020 census revealed 96% mortality of the plant population. Localities in western Guam and isolated forest fragments exhibited the greatest mortality, with 100% extirpation from two fragmented western localities. The juvenile and mature trees in Yap were unchanged from 2010 to 2018, but the seedling count was heterogeneous among the years. Constrained recruitment from seedlings to juveniles explained these dynamics. Yap transects contained about 6120 seedlings, 3400 juveniles, and 1250 mature plants per ha. Biological control of the invasive insects remains the acute conservation action needed for the Guam population. Lessons learned may be useful in other regions where invasions of non-native pests threaten biodiversity.

Three Invasive Tree Species Change Soil Chemistry in Guam Forests

Research Highlights: Established stands of Leucaena leucocephala (Lam.) de Wit, Spathodea campanulata P. Beauv., and Vitex parviflora Juss. modified soils in Guam’s limestone forests, reducing storage pools of carbon, nitrogen, and phosphorus. Background and Objectives: Invasive plants may engineer negative changes in ecosystem properties. This study was conducted to determine changes in soil chemistry following infestations of three problematic tree species on Guam. Materials and Methods: Minerals, metals, and mineralization dynamics were measured in invaded sites and paired sites with biodiverse native tree cover. Results: Most soil properties were significantly changed by long-term infestations of the invasive tree species. The soils within invaded sites exhibited total carbon, total nitrogen, and available phosphorus that were less than native sites. In contrast, the carbon/nitrogen ratio increased for every species-site combination. The other chemical properties were idiosyncratic among the sites and species. Conclusions: Mitigation and restoration activities that include the removal of these trees from project sites may require many years for the below-ground ecosystems to return to their native state. These three invasive trees decrease the ability of Guam soils to sequester recalcitrant forms of carbon, nitrogen, and phosphorus.

Stem CO2 efflux of Cycas micronesica is reduced by chronic non-native insect herbivory

Carbon dioxide efflux (E_s) from the base of Cycas micronesica K.D. Hill stems was quantified in four locations containing healthy populations and in one location with populations threatened by non-native insect herbivores in order to determine the influence of reduced plant health on E_s. Minimal variation of E_s occurred among the four locations with healthy plants, and E_s ranged from 1.68 to 1.79 µmol·m^-2·s^-1. The threatened in situ populations were on the island of Guam, where recent invasions of non-native insects have caused epidemic plant mortality, and the E_s was 0.59 µmol·m^-2·s^-1. This is the first known report of E_s for any cycad species, and the values for the unique pachycaulous stem form fit in the lower half of the range of published E_s for woody trees. The results illuminate the potential for using E_s to screen in situ C. micronesica populations to identify the individual trees with the greatest likelihood of surviving conservation measures.

Distribution of Elements along the Rachis of Cycas micronesica Leaves: A Cautionary Note for Sampling Design

Cycas micronesica K.D. Hill trees on the island of Yap were used to determine the influence of position along the leaf rachis on macro- and micro-nutrient concentrations and how leaf age affected the results. The outcomes revealed improvements to sampling protocols for future cycad leaf research. The concentration of every element except carbon and copper was influenced by leaflet position along the rachis. Most elements exhibited similar patterns for the oldest and youngest leaves on a tree, but the influence of position along the rachis for nitrogen, phosphorus, calcium, zinc, and boron was highly contrasting for old versus young leaves. The elements with the greatest variability along the rachis were potassium, phosphorus, manganese, and zinc, with the difference in basal and terminal leaflets as great as four-fold. Sampling leaflets at one position on a cycad leaf may generate inaccurate elemental concentration results for most essential nutrients other than carbon and copper. We have added position of sampled leaflets within leaves as a mandatory component of what is recorded and reported for future cycad leaf tissue analyses. Leaflets that span the full length of the rachis should be included in cycad leaf samples that are collected for tissue analysis.

Tree conservation can be constrained by agents from conservation permitting and funding agencies

Recent conservation actions for Serianthes nelsonii Merr. and Cycas micronesica K.D. Hill in the Mariana Islands have illuminated some negative consequences associated with ill-informed agents representing permitting and funding agencies. Several cases from the islands of Guam and Tinian are discussed as ineffective conservation examples, and these are countered with two examples of successful conservation approaches. When biologists that act as points of contact for federal permitting and funding agencies do not possess education, knowledge, and experience that is germane to federally listed species, sound science may be marginalized from the conservation agenda. When rapid turnover of federal conservation agents introduces dysfunction, discontinuities in collaborations may thwart success. When lapses in conservation contracts are allowed, short-term extemporary contracting approaches are utilized, and conservation practitioners that lack the ability to include an experimental approach to conservation actions are employed, the co-production of new knowledge to enable decision support tools for future decision-makers may be hindered.

Inserting cycads into global nutrient relations data sets

Global research agendas on plant nutrient relations attempt to illuminate biotic and abiotic factors that mediate nutrient relations. We contend that cycad species are not adequately represented in these global agendas. Little is known about how various cycad traits such as phylogenetics, growth form, latitudinal range, or ecological niche influence concentration, stoichiometry, and resorption dynamics of leaf nutrients. The addition of cycad species data to the global research dataset will address a critical knowledge gap and benefit global research needs to improve our systemic understanding of biotic and abiotic influences on plant nutrition.

Axial and Radial Spatial Patterns of Non-Structural Carbohydrates in Cycas micronesica Stems

The pachycaulous stem of arborescent cycad species exhibits unique traits and has received limited research. To date, nothing is known about the axial and radial spatial patterns of non-structural resources within cycad stems. Cycas micronesica K.D. Hill stem tissue was collected from apical and basal axial positions of ca. 100-cm tall plants to serve as two axial regions; and from pith, vascular, and cortex tissues to serve as three radial regions. Starch and four free sugars were quantified. These stems contained more starch than any of the individual sugars, and sucrose concentration exceeded that of fructose and glucose, which exceeded that of maltose. Total non-structural carbohydrate was least in basal vascular tissue (225 mg·g−1) and greatest in apical pith tissue (379 mg·g−1). Axial differences in NSC concentrations were negligible but radial differences were substantial. These results combine with past research to validate the non-woody cycad stem contains copious nonstructural resources available for deployment to ephemeral sinks during critical times of need.